Thymocytes and T-lymphocytes will be examined for heterogeneity in responsiveness to mitogens (PHA, ConA, allogeneic cells, LPS) and for ability to synergize with or to inhibit the responses of competent T- cells. Cells will be used from normal adult rats and mice, from animals sacrificed at intervals following lethal x-irradiation and reconstitution with syngeneic bone marrow, from similar animals reconstituted with thymocytes, and from animals simply thymectomized as adults. Subpopulations will be prepared with the use of interrupted BSA density gradients. Antigenic markers will be identified with allo or heteroantisera, and proliferative state estimated with the use of H3- thymidine on each subpopulation studied. In rats rendered tolerant by intrathymic (or systemic) injection of protein antigens (B gamma G) the competence of peripheral cells will be assayed in transfer systems or in vitro when recombined with peripheral lymphocytes from animals prepared as in A. The effect of simple thymectomy or treatment with ALS on the tolerance observed will be investigated. Active supernatants, prepared from sensitized rat lymph node cells incubated 24 hours with specific antigen will be fractionated by protein separation techniques, and the action of purified "lymphotoxin" on syngeneic fibroblast, epithelium, or endothelium monolayers evaluated by ultrastructural and enzymatic techniques.